Our studies showed that focal brain ischemia produced by one hour occlusion of the middle cerebral artery in cats produced a rise in the cerebral electrical impedance of the affected grey matter. The CEI returned approximately to pre-ischemic levels when the occlusion was released. In many of these animals a second later rise in CEI was observed to occur which appeared to be related to an increase in intracranial pressure. To test this hypothesis, brain compression was produced by epidural balloon inflation. When the epidural pressure was increased, the CEI increased as much as 216%. The regional blood flow (rCBF) was lowered but not to ischemic levels. This findings indicated that brain compression produced by edema can itself produce a reduction in extracellular space without necessarily lowering rCBF to critical ischemic levels.